Many printing systems are configured to print on both a front side and a back side of a receiver medium. Typically, a pattern (e.g., an image or text) is printed on the front side of the receiver medium using one portion of the printing system. After transportation of the receiver medium to another portion of the printing system, a second pattern (e.g., an image or text) is printed on the back side of the receiver medium.
As ink is applied to the receiver medium by the printheads of the printing system, it is absorbed by the receiver medium, and typically causes the receiver medium to expand. This expansion occurs in both in-track and cross-track directions, and often varies with position on the receiver medium. Expansion of the receiver medium often adversely affects the alignment of the receiver medium relative to the media transport of the printing system, which can lead to a reduction in print quality. Additionally, the absorption of ink by the receiver medium, often in combination with the environment in which the printing system is operated (e.g., temperature or humidity conditions), often causes the receiver medium to stretch during printing which can lead to a further reduction in print quality.
In order to achieve an acceptable level of print quality, patterns printed, for example, on the front side of a receiver medium should be properly registered with patterns printed on the back side of the receiver medium.
U.S. Pat. No. 7,295,223 to Jung, entitled “Method and apparatus for adjusting an image alignment for an image forming apparatus,” describes a method for adjusting image alignment in a printer that uses thermal printed heads to print on both sides of a medium. A first printed pattern on the first side of the medium and a second printed pattern on the second side of the medium are detected by sensor. A position deviation is determined and used to adjust print zone positions.
U.S. Pat. No. 7,394,475 to Bradley et al., entitled “Apparatus, system, and method for image registration,” describes a method of print registration which involves printing first and second registration marks. A sensor module detects registration by detecting the light transition as the first registration mark passes a first optical channel module and as the second registration mark passes a second optical channel module. In some configurations, the first and second registration marks are printed on opposite sides of the page.
U.S. Patent Application Publication 2010/0329756 to Mizes, entitled “Duplex web printer system registration technique,” describes a method for registering images printed on opposite sides of a receiver. Registration marks are printed on both sides of the receiver. A sensor on one side of the receiver is used to detect both sets of marks by transmitting light through the receiver.
Commonly-assigned U.S. Patent Application Publication 2013/0050329 to Duke et al., entitled “Registering patterns on multiple media sides,” and related U.S. Patent Application Publication 2013/0050763 to Duke et al., entitled “Multiple sided media pattern registration system,” describe a method for aligning patterns printed on both sides of a receiver. A first camera is positioned to capture an image of a first side of the media including a first printed pattern, and a second camera is positioned to capture an image of a second side of the media including a second printed pattern. A two-sided fiducial is provided adjacent to an edge of the media within the field-of-view of both cameras. Locations of the fiducial are detected in the captured images and used to define a fiducial origin in each of the images. The locations of the printed patterns are determined in each image relative to the fiducial origins, and are used to adjust the registration of subsequently printed images. This approach has the disadvantage that it requires hardware to reposition the cameras, or it requires some of the fields-of-view of the cameras to be devoted to imaging regions outside of the printed media.
There remains an ongoing need to improve the registration of patterns printed by printing systems.